The occurrence in frog skin of D-amino acid-containing peptides which function as high affinity, high selectivity ligands for mu- and delta-opioid receptors led other investigators to attempt to determine whether these, or related, peptides occur in mammalian tissues. Mor et al., reported identification of a mammalian peptide which cross-reacts with polyclonal antisera to dermorphin; however, chemical characterization of that material was not performed. Efforts are currently underway to obtain antisera which could be used in conjunction with peptide purification efforts. Upon obtaining appropriate antisera, we will attempt to test the hypothesis that dermorphin/deltorphin-related peptides occur in mammalian tissues. Formation of such peptides probably would require post- translational inversion of configuration around the alpha-carbon of the amino acid at position 2. Should it be possible to demonstrate the existence of these or related peptides In mammalian tissues, a novel form of biochemical regulation could involve the enzyme(s) which facilitates post-translational conversion of L- to D-amino acids.